Cold Pacific waters may be acting as a kind of global air conditioner — dampening the warming effects of greenhouse gases, according to a new study from the Scripps Institution of Oceanography in La Jolla.

The report, newly published in the journal Nature, could explain the mysterious plateau in global average temperatures, which have remained flat for 15 years as levels of carbon dioxide continue to rise.

“This is the first time we can account for the flattening of temperatures in a sophisticated scientific model,” said Shang-Ping Xie, a lead author of the study and the Roger Revelle Chair in Environmental Science at Scripps.

Global average temperatures had risen almost a quarter degree Fahrenheit per decade since 1950, then leveled off in the late 1990s. The pattern puzzled climate researchers, whose models projected warmer weather as carbon dioxide, the main greenhouse gas, climbed to record levels. This year, carbon dioxide peaked at a record 400 parts per million in May.

Xie and fellow Scripps researcher Yu Kosaka wondered whether the long-term ocean cooling known as the Pacific Decadal Oscillation could account for the heat hiatus, much as El Nino and La Nina cycles alter climate from year to year.

“The equatorial Pacific has been important for global climate,” Xie said. “We figured out a way to add the equatorial Pacific as an additional input to the climate model.”

When the researchers plugged Pacific sea-surface temperatures into a leading climate model, its simulations matched real-world climate observations of stalled warming, the study said. So cooler temperatures in the equatorial Pacific “drag the global curve downward,” Xie said.

That’s not likely to last, he and his colleagues said. The last period of Pacific cooling extended from the 1940s to the 1970s, said Xie, who added that such cycles typically last 20 to 30 years.

“Although similar decadal hiatus events may occur in the future, the multi-decadal warming trend is very likely to continue with greenhouse-gas increase,” according to the new report.

Other researchers have suggested that increases in particulate pollution, volcanic dust and high-altitude water vapor could diminish heat absorption. But the missing heat has continued to provoke debate, and those academic and political discussions will likely continue.

“The disagreement of that (temperature) curve, with the observations for the past 15 years, has caused a big stir in the media and political circles,” Xie said.

Factoring in the Pacific water temperatures may answer another puzzle, he said.

While global average temperatures have flattened since the late 1990s, summer heat has spiked, producing record heat waves in Europe, Russia, the United States and China over the past decade, Xie said.

“All that record heat you heard in the news seemed to be contradictory to the flat global average temperate,” Xie said. “So we eventually figured out, the global average temperature change has a pronounced seasonal cycle to it.”

To further test such findings, Xie aims to conduct similar simulations using ocean temperature measurements dating back to the 1880s — the earliest period for which such data is available. And he said other researchers will probably see whether they can reproduce his findings using other climate models.

“I think it is an excellent study,” said Venkatachalam Ramaswamy, director of the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory in Princeton, N.J., which produced the climate model that Xie and Kosaka adapted for their study. “The way they did the experiments is fairly original and has novel outcomes. It offers substantive input into the physical factors that might explain why the rate of warming has decreased over the last decade.”

Xie said he was curious about the global warming hiatus, and now is excited to have found an answer for his curiosity.

“For scientists, it’s more interesting to find contradictions, and it’s really a thrill to be able to resolve some of the contradictions,” Xie said.